1. Field of the Invention
The present invention relates to an ultrasonic transducer for use in a fluid medium.
2. Description of Related Art
Various types of ultrasonic transducers for use in fluid media are known from the related art. By ultrasonic transducer one generally means an element that is in a position to emit ultrasonic signals into the fluid medium and/or to pick up ultrasonic signals from the fluid medium and convert them to electrical signals, for example. Such ultrasonic transducers are used, for instance, in ultrasonic flowmeters in industrial process engineering and/or in the automotive field, for example, in flowmeters (ultrasonic flowmeters, UFM), particularly in the intake tract and/or the exhaust tract of internal combustion engines. Examples of such ultrasonic transducers are known from published German patent application documents DE 10 2007 010 500 A1, DE 10 2007 037 088 A1 or from German patent application DE 10 2008 055 126.0. The ultrasonic transducers described there may also basically be used within the scope of the present invention, and modified according to the present invention.
As a rule, ultrasonic transducers have at least one electroacoustic element, for instance, a piezoelectric transducer. However, in flow metering in air or other gases, the problem arises that a high acoustical impedance difference will be registered between the material of the electroacoustic transducer element and the air, which leads to high coupling losses in the coupling of ultrasonic signals between the electroacoustic transducer and the gas. The signal level swings may be correspondingly weak if, for example, from the transducer signals during flow metering, air measurement signals within the system control of an internal combustion engine are derived. Thus, ultrasonic transducers are familiar which have sound-radiating resonance members or matching members, such as a metallic diaphragm or a λ/4-impedance-matching layer. Such impedance-matching layers, which may basically be geometrically designed as desired, are known, for instance, from the abovementioned documents of the related art.
In known ultrasonic transducers, a housing is usually used, in which a transducer core is accommodated. On the side facing the fluid medium, this housing has an opening, inside which the radiation surface of the transducer core is situated, via which ultrasonic signals are emitted or picked up. In order to close the housing in a manner that is sealed to the medium and/or pressure tight, it is known from German patent application document DE 10 2008 055 126.0 that one may span this opening entirely or partially using a sealing foil. However, the technical challenge comes up in this context that, at the edges of the sealing foil, media continue to be able to penetrate into the inner space of the housing, which can make itself known disadvantageously during flow measurements in internal combustion engines, for example.